Interference and fading are significant impediments to achieving high data rates in today's wireless communication systems. Given the tendency for channel conditions to sporadically and significantly fade, communication resources are conservatively allocated, leaving excessive amounts of communication resources unused most of the time. Efforts to combat the impact of fading include incorporating transmission diversity or controlling modulation and coding techniques in relation to channel conditions.
Spatial diversity is typically a function of the number and placement of transmit and receive antennas relative to a transmitter and receiver. Systems employing spatial diversity with multiple transmit and receive antennas are generally referred to as multiple-input multiple-output (MIMO) systems. Accordingly, a transmitting device will have N transmit antennas, and the receiving device will have M receive antennas. Space-time coding controls what data is transmitted from each of the N transmit antennas. A space-time encoding function at the transmitter processes data to be transmitted and creates unique information to transmit from the N transmit antennas. Each of the M receive antennas will receive signals transmitted from each of the N transmit antennas. A space-time decoding function at the receiving device will combine the information sent from the N transmit antennas to recover the data.
Space-time encoding is typically implemented using one of two techniques. The first technique encodes the same data in different formats for transmission from the different transmit antennas. Thus, the same data is transmitted in different formats from each of the N transmit antennas. The second technique transmits different data from different ones of the N transmit antennas wherein the redundancy of the second technique is avoided. The first technique, which is often referred to as space-time transmit diversity (STTD), is effective in maximizing diversity but inefficient due to the requisite redundancy. The second technique, which is often referred to as V-BLAST (Vertical—Bell Laboratories Layered Space Time), increases system throughput for systems having sufficient diversity available. Once a threshold amount of diversity is achieved, data rates increase linearly with the number of transmit and receive antennas for BLAST systems, whereas additional spatial diversity has little impact on data rates in STTD systems. Thus, STTD and BLAST systems have unique advantages and disadvantages. Historically, STTD and BLAST spatial encoding techniques have not been employed in the same system. Further information related to STTD and V-BLAST can be found in Siavash M. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications,” IEEE J. Select. Areas Commun., vol. 16, pp. 1451–1458, Oct. 1998; G. J. Foschini, “Layered Space-time Architecture for Wireless Communications in a Fading Environment when Using Multi-element antennas,” Bell Labs Tech. J., pp. 41–59, Autumn 1996; G. D. Golden, G. J. Foschini, R. A. Valenzuela, and P. W. Wolniansky, “Detection Algorithm and Initial Laboratory Results Using V-BLAST Space-time Communication Architecture,” Electronics Letters, vol. 35, pp 14–16, January 1999; and P. W. Wolniansky, G. J., Foschini, G. D. Golden, and R. A. Valenzuela, “V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-scattering Wireless Channel,” Proc. IEEE ISSSE-98, Pisa, Italy, September 1998, pp. 295–300, which are incorporated herein by reference.
Prior to space-time coding, data to be transmitted is encoded to facilitate error correction and modulated or mapped into symbols using any number of available modulation techniques, such as quadrature phase shift keying (QPSK) and x-quadrature amplitude modulation (QAM). The type of encoding for error correction and modulation techniques greatly influences the data rates, and their applicability is a function of channel conditions.
Since STTD and BLAST space-time coding techniques are not traditionally combined, many systems are unable to take full advantage of available resources even when error encoding and modulation techniques are controlled based on channel conditions. Over time, the optimal space-coding technique in a communication environment is likely to change between STTD and BLAST. Further, for either STTD or BLAST space-time coding, the optimal type of error encoding or modulation techniques will also change over time. As such, there is a need to adaptively control space-time coding, error rate encoding, and modulation techniques to optimize communication efficiency between communicating devices over varying channel conditions.